Controls to low sulphidation epithermal Au-Ag mineralisation

Presentation to the Sydney Mineral Exploration Discussion Group

Greg Corbett

PO Box 282 Willoughby NSW Australia - July 2007, updated 2008 & 9.

greg@corbettgeology.com

Much of the mineralisation in low sulphidation epithermal vein deposits occurs within ore

shoots which are best developed at the coincidence of several controls described below.

However, it is first interesting to consider the fluids which form many veins.

Figure 1. Model to account for varying hydrothermal fluids which contribute towards the

development of banded low sulphidation epithermal Au-Ag veins containing many varied

vein elements.

Low sulphidation epithermal Au-Ag deposits are distinguished from high sulphidation

deposits primarily by the different sulphide mineralogy (pyrite, sphalerite, galena,

chalcopyrite) typically within quartz veins with local carbonate, and associated near

neutral wall rock alteration (illite clays), deposited from dilute hydrothermal fluids

(Corbett and Leach, 1998). Many low sulphidation veins are well banded and each band

represents a separate episode of hydrothermal mineral deposition. Consequently recent

thinking separates varying styles of rising hydrothermal fluids (figure 1) which contribute

towards low sulphidation vein formation as:

Meteoric dominated waters commonly form shallow circulating cells and deposit

clean quartz, which has not come in contact with buried intrusion sources for

metals and so are commonly barren.

Magmatic-meteoric waters developed where meteoric waters circulate to

sufficiently deep crustal levels to come in contact with magmatic sources for

metals and so contain low grade mineralisation within disseminated sulphides.

Magmatic dominant waters have been derived from intrusion sources for metals at

depth and so contain highest precious metal values associated with sulphides.

Controls to ore shoot development in low sulphidation epithermal deposits might be

considered as:

Lithological control occurs mainly as competent or brittle host rocks which develop

throughgoing fractures as vein hosts, although permeability is locally important. In

interlayered volcanic sequences epithermal veins may be confined to only the competent

rocks while the intervening less competent sequences host only fault structures.

Figure 2. Illustration of the structural control to ore shoot formation in different structural

environments and associated ore shoot orientations.

Structures act as fluid channelways and more dilational portions of the host structures

may represent sites of enhanced fluid flow and so promote the development of ore shoots

which host most mineralisation in many low sulphidation vein systems (Corbett 2002a).

Here veins are wider and exhibit higher metal grades evident on gram x metre plots.

Elsewhere fault intersections host ore shoots at sites of fluid mixing. Several structural

settings provide ore shoots of varying orientations (figure 2). Steep dipping strike-slip

structures provide vertical ore shoots in flexures and fault jogs. Although tension veins

and dilatant sheeted veins dominate as ore hosts splays and pull-apart basins are also

recognised. Vertical zonation is apparent as negative flower structures in these ore

environments which extend from splay where porphyry intrusions are localised at greatest

depths, to overlying tension veins and and surficial pull-apart basins where mineralisation

is localised by the normal faults (figure 3; Corbett and Leach, 1998). Normal, and in

particular listric faults, in extensional settings host wider and higher grade veins as flat

ore shoots in steep dipping vein portions of those structures. In compressional settings

reverse faults host flat plunging ore shoots in reverse faults.

Figure 3. Negative flower structure illustrating vertical zonation in strike slip dilatant ore

settings, from Corbett and Leach (1998).

Figure 4. Conceptual model illustrating styles of magmatic arc porphyry Cu-Au and

epithermal Au-Ag mineralisation.

Styles of low sulphidation Au are distinguished (Corbett and Leach, 1998; Corbett 2002b,

2004, 2005) according to mineralogy and relation to intrusion source rocks and influence

precious metal grade, Ag:Au ratio, metallurgy and Au distribution (figure 4).

Quartz-sulphide Au + Cu mineralisation is characterised by quartz and pyrite as

the main sulphide, although lower temperature marcasite, and higher temperate

pyrrhotite and chalcopyrite are also recognised (Nolans, Adelong, Mineral Hill,

Round Mountain, Emperor). Quenched very fine pyrite is commonly arsenean and

locally displays difficult metallurgy (Lihir), while coarser sulphides are typically

associated with near surficial supergene Au enrichment.

Carbonate-base metal Au deposits overprint quartz-sulphide Au, display higher

Au contents, increased Ag:Au ratios, with additional sphalerite greater than

galena, and an important carbonate component, described below (Cowal, Porgera,

Kelian, Acupan, Antamok). These deposits are the most prolific Au producers in

the SW Pacific rim, although with locally quite irregular Au distribution,

commonly as stock work and sheeted veins or breccia matrix, including in

association with phreatomagmatic breccias.

Polymetallic Ag-Au deposits dominate in the Americas as fissure vein Ag-rich

equivalents to carbonate-base metal Au deposits (Fresnillo, Palmarejo). In

dilational structural settings these pass upwards to chalcedony-ginguro deposits

and commonly display vertical metal zonation from Cu rich at depth to higher

level Ag (figure 5).

Figure 5. Conceptual model showing setting of many polymetallic Ag-Au veins in steep

dipping portions of listric faults and vertical metal zonation, grading to toxic elements

which may overlie economic mineralisation.

Epithermal quartz Au-Ag deposits are characterised as Ag-poor often bonanza Au

grades, developed greatest distances from magmatic source rocks, in association

with only minor quartz, illite, chlorite and local pyrite gangue, and so can be

difficult to identify. They contribute to irregular Au distribution in overprinted

carbonate-base metal and quartz-sulphide Au deposits (Porgera Zone VII,

Emperor) and may be Te-bearing.

Chalcedony-ginguro epithermal Au-Ag deposits commonly display bonanza Au

grades and occur as generally Ag-rich banded veins comprising chalcedony,

adularia, quartz pseudomorphing platy calcite and ginguro black sulphidic

material described by 19th

century Japanese miners. While much of the gangue

may be deposited from boiling meteoric dominant waters most high grade Au

mineralisation occurs in the magamtically-derived ginguro bands deposited from

rapidly cooling fluids, locally aided by mixing with ground waters.

Mechanisms of Au deposition have a profound effect upon Au grade and commonly

account for the development of bonanza Au grades (figure 1; Leach and Corbett, 2008)

varying from:

Cooling in the case of many coarse sulphides with low grade Au contents.

Rapid cooling promoted by quenched magmatic fluids evidenced by fine

sulphides, or by mixing of ore fluids with deep circulating meteoric waters,

commonly recognised in high precious metal polymetallic vein deposits where

low temperature quartz (opal) is in contact with high temperature sulphides.

While boiling fluids deposit much of the gangue (adularia, quartz pseudomorphing

platy calcite and local chalcedony), in epithermal veins and Au, other mechanisms

might also be considered to account for elevated Au grades.

Mixing of oxygenated ground waters with ore fluids at elevated crustal settings

produces elevated Au grades and is evidenced by hypogene haematite in the ore

assemblage (figures 1 & 4).

Mixing of bicarbonate waters derived from the condensation of CO2 volatiles

released from cooling intrusions is responsible for the development of higher Au

grades as the carbonate-base metal group of low sulphidation Au deposits(figures

1 & 4).

Mixing of low pH waters, developed by the condensation of H2S volatiles above

the water table, and responsible for the development of near surficial acid sulphate

caps, provide the highest Au grades and is evidenced by the presence of hypogene

kaolin including halloysite within the ore assemblage(figures 1 & 4).

References cited

Corbett, G.J., 2002a, Epithermal Gold for Explorationists -Australian Institute of

Geoscientists Presidents lecture: AIG News No 67, 8p.

Corbett, G.J., 2002b, Structural controls to Porphyry Cu-Au and Epithermal Au-Ag

deposits in Applied Structural Geology for Mineral Exploration, Australian Institute of

Geoscientists Bulletin 36, p. 32-35.

Corbett, G.J., 2004, Epithermal and porphyry gold – Geological models in Pacrim

Congress 2004, Adelaide, The Australasian Institute of Mining and Metallurgy, p. 15-23.

Corbett, G.J., 2005, Epithermal Au-Ag deposit types – implications for exploration:

Proexplo Conference Peru May 2005, published on CD.

Corbett, G. J., and Leach, T. M., 1998. Southwest Pacific rim gold-copper systems:

structure, alteration, and mineralisation. Society of Economic Geologists Special

Publication 6, 234 p.

Leach, T.M. and Corbett, G.J., 2008, Fluid mixing as a mechanism for bonanza grade

epithermal gold formation: Terry Leach Symposium, Australian Institute of Geoscientists,

Bulletin 48, p. 83-92.